1. Field of the Invention
The present invention relates to a pixel circuit including an organic EL (Electroluminescence) light emitting element or the like, an active matrix type display device, and a method of manufacturing the pixel circuit.
2. Description of the Related Art
In an image display device, for example a liquid crystal display, a large number of pixels are arranged in the form of a matrix, and light intensity is controlled for each pixel according to the information on the image to be displayed.
While the same is true for an organic EL display or the like, the organic EL display is an emissive display having a light emitting element in each pixel circuit. This gives advantages of providing higher image viewability, eliminating a need for a backlight, and having higher response speed, for example, as compared with the liquid crystal display.
In addition, the organic EL display differs greatly from the liquid crystal display or the like in that the luminance of each light emitting element is controlled by the value of a current flowing through the light emitting element, and thereby color gradation is obtained, that is, the light emitting element is of a current-controlled type.
As in the liquid crystal display, there is a simple matrix system and an active matrix system as possible driving systems of the organic EL display. The simple matrix system has a simple structure, but presents problems including for example a difficulty in realizing a large high-definition display. Therefore the active matrix system, which controls a current flowing through a light emitting element within a pixel circuit by an active element, or typically a TFT (Thin Film Transistor), provided within the pixel circuit, has been actively developed.
FIG. 1 is a block diagram showing a configuration of an ordinary organic EL display device.
As shown in FIG. 1, this display device 1 includes: a pixel array unit 2 having pixel circuits (PXLC) 2a arranged in the form of an m×n matrix; a horizontal selector (HSEL) 3; a write scanner (WSCN) 4; signal lines (data lines) SGL1 to SGLn selected by the horizontal selector 3 and supplied with a data signal corresponding to luminance information; and scanning lines WSL1 to WSLm selected and driven by the write scanner 4.
Incidentally, the horizontal selector 3 and the write scanner 4 may be formed on polycrystalline silicon, or formed by a MOSIC or the like on the periphery of the pixels.
FIG. 2 is a circuit diagram showing an example of configuration of a pixel circuit 2a in FIG. 1 (see U.S. Pat. No. 5,684,365 and Japanese Patent Laid-Open No. Hei 8-234683, for example).
The pixel circuit of FIG. 2 has the simplest circuit configuration among the large number of circuits that have been proposed, called the two-transistor driving system circuit.
The pixel circuit 2a of FIG. 2 includes a p-channel thin film field effect transistor (hereinafter referred to as a TFT) 11 and a TFT 12, a capacitor C11, and an organic. EL light emitting element (OLED) 13 as a light emitting element. In FIG. 2, SGL denotes a signal line, and WSL denotes a scanning line.
The organic EL light emitting element has a current rectifying property in many cases, and may therefore be referred to as an OLED (Organic Light Emitting Diode). Although the symbol of a diode is used for a light emitting element in FIG. 2 and other figures, the current rectifying property is not necessarily required for OLED, as described in the following description.
In FIG. 2, the source of the TFT 11 is connected to a power supply potential Vcc. The cathode of the light emitting element 13 is connected to a ground potential GND. The operation of the pixel circuit 2a of FIG. 2 is as follows.
Step ST1:
When the scanning line WSL is set in a selected low state, and a writing potential Vdata is applied to the signal line SGL, TFT 12 conducts to charge or discharge the capacitor C11, and the gate potential of the TFT 11 becomes the writing potential Vdata.
Step ST2:
When the scanning line WSL is set to a non-selected high state, the signal line SGL and TFT 11 are electrically disconnected from each other. However, the gate potential of the TFT 11 is maintained stably by the capacitor C11.
Step ST3:
A current flowing through the TFT 11 and the light emitting element 13 has a value corresponding to the gate-to-source voltage Vgs of the TFT 11, and the light emitting element 13 continues emitting light at a luminance corresponding to the current value.
An operation of selecting the scanning line WSL and transmitting the luminance information supplied to the data line to the inside of the pixel as in the above-described step ST1 will hereinafter be referred to as “writing.”
As described above, once the writing potential Vdata is written in the pixel circuit 2a of FIG. 2, the light emitting element 13 continues emitting light at a constant luminance until the writing potential Vdata is next rewritten.
As described above, in pixel circuit 2a, the value of the current flowing through the light emitting element 13 is controlled by changing the voltage applied to the gate of the TFT 11 as driving (drive) transistor.
At this time, the source of the p-channel drive transistor is connected to the power supply potential Vcc, and the TFT 11 operates in a saturation region at all times. The TFT 11 is therefore a constant-current source having a value expressed by the following Equation 1.(Equation 1)Ids=½·μ(W/L)Cox(Vgs−|Vth|)2  (1)where μ denotes carrier mobility, Cox denotes gate capacitance per unit area, W denotes gate width, L denotes gate length, Vgs denotes the gate-to-source voltage of the TFT 11, and Vth denotes the threshold voltage of the TFT 11.
Each light emitting element in a simple matrix type image display device emits light only at the moment when the light emitting element is selected. On the other hand, the light emitting element in the active matrix system continues emitting light even after writing has ended, as described above. The active matrix system is therefore advantageous especially in a large high-definition display in that the peak luminance and the peak current of the light emitting element can be decreased as compared with the simple matrix system.
FIG. 3 is a diagram showing a secular change in the current-voltage (I-V) characteristic of the organic EL light emitting element. In FIG. 3, the curve represented as a solid line indicates a characteristic at a time at the initial state, and the curve represented as a broken line indicates the characteristic after a secular change.
Generally, as shown in FIG. 3, the I-V characteristic of the organic EL light emitting element is degraded with the passage of time.
However, because the two-transistor driving of FIG. 2 is constant-current driving, a constant current continues to flow through the organic EL light emitting element as described above. As a result, the light emission luminance of the organic EL light emitting element is not degraded with time even when the I-V characteristic of the organic EL light emitting element is degraded.
The pixel circuit 2a of FIG. 2 is formed with p-channel TFTs. When the pixel circuit 2a of FIG. 2 is formed with n-channel TFTs, an existing amorphous silicon (a-Si) process is used in TFT fabrication, thereby reducing the cost of a TFT substrate.
Next, the basic pixel circuit in which the transistors are replaced with n-channel TFTs will be described.
FIG. 4 is a circuit diagram showing a pixel circuit in which the p-channel TFTs in the circuit of FIG. 2 are replaced with n-channel TFTs.
The pixel circuit 2b of FIG. 4 includes n-channel TFTs 21 and 22, a capacitor C21, and an organic EL light emitting element (OLED) 23 as a light emitting element. In FIG. 4, SGL denotes a data line, and WSL denotes a scanning line.
In this pixel circuit 2b, the drain side of the drive transistor TFT 21 is connected to a power supply potential Vcc, and the source of the TFT 21 is connected to the anode of the EL light emitting element 23, whereby a source follower circuit is formed.
FIG. 5 is a diagram showing an operating point of the TFT 21 as a drive transistor and the EL light emitting element 23 in an initial state. In FIG. 5, the horizontal axis indicates the drain-to-source voltage Vds of the TFT 21, and the vertical axis indicates the drain-to-source current Ids of the TFT 21.
As shown in FIG. 5, source voltage is determined by the operating point of the TFT 21 as drive transistor and the EL light emitting element 23, and the voltage has a different value depending on gate voltage.
Because the TFT 21 is driven in the saturation region, TFT 21 passes the current Ids having the current value of the equation shown above in Equation 1, relating to the gate-to-source voltage Vgs corresponding to the source voltage at the operating point.